Method of epitaxial deposition wherein spent reaction gases are added to fresh reaction gas as a viscosity-increasing component



May 20, 1969 s L 3,445,300

METHOD OF EPITAXIAL DEPOSITION WHEREIN SPENT REACTION GASES ARE ADDED TO FRESH REACTION GAS AS A VISCOSITY-INCREASING COMPONENT Filed Feb. 1, 1966 Fig.1 17 1 L 1311 4 N 5* 8 1 18 13 111 5 t5 g 20 A1 1 ill L F232 i F|3g1.3 L 4 T- L i 1\\\\ 1 1 &Y\ 2 1 P f 1= Z 11 12 m F|g.4 m Fig.5 1.1 51 l zq AVQW fimN 11 m\ dfdz 11 11 1 12 United States Patent Int. 01. Hon 7/36 US. Cl. 148175 6 Claims ABSTRACT OF THE DISCLOSURE In the method of growing uniform epitaxial layers of semiconductor material, particularly silicon, by pyrolytic dissociation of a gaseous compound of the semiconductor material and precipitating the evolving semiconductor material upon a heated monocrystalline substrate, the improvement which comprises admixing partly spent residual gases of the pyrolytic reaction to fresh reaction gas, whereby said residual gases serve as a viscosityincreasing component. Other disclosed viscosity increasing components include nitrogen or a gas from the argon group. The spent residual gases may be added from the beginning of the deposition reaction or after a period of time has elapsed. A dopant material may be added to the reaction gas and its concentration varied during the course of the reaction.

My invention relates to a method of growing uniform epitaxial layers of semiconductor material, especially but not exclusively silicon, by pyrolytically dissociating a gaseous compound of the semiconductor material, and precipitating the evolving semiconductor material upon a heated monocrystalline substrate preferably consisting of the same material.

With the known methods of this type it has been found difiicult to maintain the epitaxially grown film or layer within the desired narrow thickness tolerance. Considerable fluctuations in layer thickness are caused particularly by the fact that at the beginning of the precipitation process the fresh reaction gas is supersaturated with semiconductor material. This manifests itself disturbingly, particularly in reduction processes occurring in the excessive presence of hydrogen, in that the dissociation or elimination of material does not take place uniformly at all localities of the reaction surface. Within the processing equipment, therefore, differently thick precipitations will occur, depending upon the flow profile and flow direction. These differences in thickness may be observed from wafer to wafer when simultaneously processing several substrate wafers, or the differences in thickness may even be present within a single wafer. So far, no devices for performing epitaxial precipitation methods have become known in which, by means of nozzle constructions or similar flow control devices, the above-mentioned disadvantage has been completely eliminated.

It is therefore an object of my invention to devise a method which reliably secures a production of epitaxially grown layers or films of semiconductor material that are free from the non-uniformities in layer thickness heretofore encountered.

To this end, and in accordance with my invention, I modify the above-mentioned pyrolytic dissociation and precipitation process by admixing to the reaction gas a viscosity-increasing additional component whose molar 3,445,300 Patented May 20, 1969 weight is considerably higher than that of hydrogen, preferably amounting to a multiple of the hydrogen molar weight, the added component being chosen from gaseous materials that are inert with respect to the pyrolytic reaction.

Applicable as a reaction gas, for example, is a mixture of a halogen compound or a hydrogen-halogen compound of the semiconductor material to be precipitated, this compound being used in mixture with hydrogen. Thus, for producing epitaxial layers of silicon, silicochloroform may be used. In each case it is necessary to provide for an adaptation of the molar ratio between the two reaction partners. Suitable as viscosity-increasing addition to the reaction gas are gases, such as nitrogen, argon or krypton, that are inert at the reaction temperature. Also suitable as added inert and viscosity-increasing components are the partially spent residual gases resulting from the method according to the invention. According to a particular embodiment of the present invention, such residual gases are recycled back to the fresh-gas supply in order to increase the viscosity of the reaction-gas mixture applied to the heated substrate.

When using silicochloroform as gaseous semicon ductor compound, it has been found favorable to provide for a molar mixing ratio of silicochloroform to hydrogen in the range from about 0.01 to 0.1. The amount of the added component may then be approximately 5 to 50 mole percent, preferably 20 to 30 mole percent, of the hydrogen quantity being supplied.

The additional component may be admixed to the reaction gas immediately from the beginning of the pyrolytic process. Another mode of performing the method of the invention is to add the viscosity-increasing component only at a subsequent stage, preferably after a preceding annealing process. The latter mode is particularly advantageous when using nitrogen as highviscosity component.

According to still another feature of the invention, the reaction gas may be given an addition of doping substance. The quantity of the admixed doping material may be kept constant during the course of the reaction or it may be varied during the reaction.

Semiconductor material made by the method according to the invention is particularly favorable for the production of electronic semiconductor devices such as transistors, rectifiers or the like. By virtue of the uniformity of the epitaxial layers, the substrates provided with these layers can be subjected to further fabrication into semiconductor devices virtually without mechanical machining of the layers.

Further details will be apparent from the following description of embodiments illustrated by way of example on the accompanying drawings in which:

FIG. 1 shows schematically and partly in section an apparatus for performing the method of the invention.

FIG. 2 shows schematically a cross section of a silicon wafer made according to the invention; and FIG. 3 shows schematically and for comparison a cross section typical of products resulting from known methods.

FIGS. 4 and 5 respectively show in cross section two groups of semiconductor discs made by the method of the invention and by prior methods respectively.

The apparatus illustrated in FIG. 1 comprises a reaction vessel 1 of quartz or quartz glass in which a substrate body 2 of n-type silicon is placed upon the top of a heater 3. The electric leads 4 and 5.of the heater 3 are connected to respective terminals 6 and 7 on the outside of the reaction vessel for attachment to a voltage source. The vessel 1 has an inlet 8 for supplying the reaction-gas mixture, and an outlet nipple 9 through which the residual gases leave the vessel.

During performance of the process, the vessel is supplied with a reaction-gas mixture entering in the direction of the arrow 10. The mixture is composed of vaporous semiconductor compound, hydrogen and an addition of argon. In the example here described, silicochloroform is employed as the semiconductor compound to be dissociated. The gaseous compound is obtained by evaporating the liquid compound in an evaporator vessel 11 located within a temperature control bath 110. Hydrogen is introduced into the evaporator vessel 11 from a storage bottle 13 past an overpressure relief valve 130. Simultaneously with the hydrogen, the additive argon is introduced from a storage bottle 14 communicating with another overpressure relief valve 140. Control valves 15 and 16 for hydrogen and argon permit selectively a simultaneous or successive supply of these two gases. A cooling trap 12 connected in the gas path ahead of evaporator vessel 11 removes any liquid from the entering gases. Flow meters 17 and. 18 indicate the quantities of the respective gases being supplied. The apparatus is further equipped with stop valves 19 and 20 with Whose aid the reaction vessel 1 can be sealed off.

In the processing example here described, a mixture of silicochloroform and hydrogen in a. molar ratio of 0.3 is being used. Added to this mixture are about mole percent argon. This mixture enters in the direction of the arrow 10 into the reaction vessel 1 where it is dissociated at the substrate 2 heated to a temperature of 1130 C. The evolving silicon precipitates onto the substrate 2.

The epitaxial layer thus grown on the substrate surface exhibits an extremely uniform constitution. The layer thickness is virtually independent of the flow direction of the gas. If several substrate Wafers are simultaneously processed instead of only one substrate, they show virtually no dilference in layer thickness between each other. This result is represented in FIGS. 2, 3 and 4, 5.

FIG. 2 shows schematically and by way of example a silicon circular disc 21 made by the method of the invention. For the purpose of illustration, the thickness values d d in FIG. 2 and the corresponding values d and 11 in FIG. 3 are greatly exaggerated as to absolute dimensions and their ratio to the disc diameter. Denoted by d is the layer thickness at the side where the fresh gas first reaches the semiconductor body 21. There is virtually no difference between the thickness al and the thickness d In contrast, a semiconductor disc 31 as shown in FIG. 3, which is made by the conventional method without the addition of a viscosityincreasing component, has a much larger thickness d at the incoming side of the gas flow than at the opposite side.

FIGS. 4 and 5 represent the analogous conditions obtaining with a simultaneous production of several semiconductor discs which are denoted by 41 in FIG. 4 and by 51 in FIG. 5. In this case, too, the semiconductor discs made by the method of the invention have equal layer thicknesses d and d regardless of the particular locality of the substrates, whereas the conventional method results in semiconductor discs whose respective thicknesses d and 0. exhibit considerable differences.

Corresponding results are obtained when using nitrogen in lieu of argon. It must be taken into account, however, that nitrogen may convert silicon to silicon nitride at reaction temperatures in the vicinity of 1300 C. At temperatures below 1300 C., the danger of nitride formation is not very large because at this temperature the nitrogen molecule is very stable and requires a correspondingly high activating energy for entering into chemical reactions. At temperatures above 1300 C., the formation of silicon nitride is disturbingly manifested by the fact that silicon nitride, once formed, can only with difficulty be dissociated by increase in temperature. It is similarly inadvisable to employ nitrogen activated in any other manner.

The addition of inert gases having a molar weight considerably larger than that of hydrogen and being preferably a multiple of the hydrogen molar weight or in a higher order of magnitude (hydrogen=2, argon=39.9, N =28), has the advantage that, by greatly increasing the viscosity, the diffusion speed of the re action gas is greatly decreased. It is essential, however, that the added viscosity-increasing gas does not participate in the chemical conversion of the reaction partners themselves. For example, if hydrogen chloride (molecular weight 36.5) were added, it would act not only compensating in the sense of retarding the diffusion, but would also directly participate in the chemical equilibrium of the silicon precipitation.

As mentioned, the residual gases resulting from the dissociation reaction, which leave the reaction vessel by being only partly subjected to chemical conversion, may also be employed as additional viscosity-increasing components. For this purpose, the spent waste gases from the reaction vessel are cycled back into the reaction vessel so that they again enter into the dissociation-reaction space together with the fresh reaction gas. This expedient, however, is not by far as generally applicable as the addition of heavy gases that do not participate in the reaction, such as nitrogen at temperatures below 1300 C., or generally argon and other noble gases.

I claim:

1. In the method of growing uniform epitaxial layers of semiconductor material, particularly silicon, by pyrolytic dissociation of a gaseous compound of the semiconductor material and precipitating the evolving semiconductor material upon a heated monocrystalline substrate, the improvement which comprises admixing partly spent residual gases of the pyrolytic reaction to fresh reaction gas, whereby said residual gases serve as a viscosity-increasing component.

2. The method according to claim 1, wherein the reaction gas is a mixture of silicochloroform and hydrogen in a molar mixing ratio between 0.01 and 0.1.

3. The method according to claim 2, wherein the amount of said spent residual gases is about 20 to about 30 mole percent of the hydrogen quantity.

4. The method according to claim 1, which comprises adding said spent residual gases from the beginning of the pyrolytic reaction.

'5. The method according to claim 1, which comprises adding dopant material to the reaction gas.

6. The method according to claim 1, which comprises adding dopant material to the reaction gas, and varying the quantity of the dopant material during the course of the reaction.

References Cited UNITED STATES PATENTS 2,910,394 10/1959 Scott et al 148-175 3,152,933 10/1964 Reuschez 148175 3,173,814 3/1965 Law 148-l75 3,197,411 7/1965 Frosch 148175 XR 3,200,018 8/1965 Grossman 148175 3,297,501 1/1967 Reisrnan 148-474 3,354,004 11/1967 Reisman et al o- 148175 3,382,113 5/1968 Ebert et al 117106 XR L. DEWAYNE RUTLEDGE, Primary Examiner.

P. WEINSTEIN, Assistant Examiner.

US. Cl. X.R. 

